Measurement of pressure is very vital in industrial manufacturing and processing. Particularly measurement of pressure with accuracy over a wide range is needed in such industries as automobiles, aerospace, steel and for synthesis of high strength materials. In all these industrial sectors, the accuracy in measurement is of paramount importance not only due to quality considerations but also to safety requirements. No single gadget can measure the entire pressure range with the same accuracy and reproducibility. The gadgets may also not be sensitive enough to small changes in pressure and be stable over a wide working temperature (in the range of 10-50° C.). A system is therefore required which will have the necessary characteristics of large pressure coefficient to detect small changes even in a large absolute value and have a minimum drift over a large temperature range i.e. have a low temperature coefficient.
Pressure measurements have traditionally been made using a liquid column manometer. While this serves as an absolute instrument, its use is limited to lower ranges for pressure of 0.1 Pa to 200 kPa. Another disadvantage of this device is that it cannot be transported easily from one place to another. P. L. M. Heydemann and B. E. Welch, et al in ‘Experimental Thermodynamics’, (Vol. II, B. LeNiendre and B. Vodar (eds), Butterworths (1975)), R. S. Dadson, et al in ‘The Pressure Balance: Theory and Practice’, National Physical Laboratory, Teddington, England and J. K. N. Sharma and Kamlesh K. Jain, Pramana, J Phys Vol 27 pp 417 (1986) disclose that pressures up to 300 MPa can be measured easily by piston gauges and that these piston gauges can be transported after taking certain precautions. However, these piston gauges cannot be used for pressures beyond 300 MPa without increasing the size of the entire assembly thereby making it cumbersome to use even with trained manpower. As a result this device is useless for fieldwork.
G. F. Molinar and L. Bianchi and J. K. N. Sharma, et al disclose the use of manganin resistance wires to sense pressures over a wide range. The main drawback of manganin resistance wires is the low accuracy of just ±0.1% when the requirement normally is of at least ±0.05% or better. Further this sensor has the undesirable property of zero shift with time leading to erroneous measurements and needs stringent temperature control during measurement. While this device may be useful for high-pressure work, the use for low pressure ranges like 58 Mpa is limited. In order to cover lower ranges one necessarily has to use another device.
Another pressure measuring device is disclosed by A. W. Birks (Report No. 1566 of Queen's University of Belfast). This disclosure describes the device as a Strain Gauge. However, this device also suffers from the same drawbacks as for manganin wire. Further the accuracy in pressure measurement of this device is low due to large hysteresis and zero shift.
Yet another type of a pressure measuring device based on resistance measurement has been disclosed in a U.S. Pat. No. 5,578,765. The said patent disclosure teaches that the pressure due to an applied force on a transducer array, essentially consisting of resistive elements leads to a change in the resistance value when the applied force is changed. The dependence of pressure is related to gradual touching of the two arrays thereby decreasing the resistance of the system. The inventors have disclosed curvilinear relation between the measured resistance and the applied pressure. At high pressures, the resistance drops to fairly low values. This low resistance values may not be measurable so accurately thereby leading to possible errors in pressure measurement. Another drawback is that the device of this patent needs a threshold pressure for it to act as a pressure sensor. As a result, the use of this device is limited in respect of pressures lower than the required threshold value.
G. F. Molinar, et al in 1998 attempted to use a ceramic rod to improve upon the existing pressure sensor (Measurement, Vol. 24, pp 161 (1998)). While this pressure transducer had improved resolution and sensitivity, it lacked repeatability and had marked hysteresis. The presence of last property is undesirable as this leads to increased error in the measured pressure.
PCT Application PCT/WO US9405313 discloses a capacitive transducer that can measure pressure from as low as 100 PSI to 22,000 PSI. However, the structure used is rather complicated—a metal diaphragm is separated from a dielectric alumina by as small a distance as 0.00005 inch and 0.020 inch. This small distance between the metal diaphragm and the insulator disc is difficult to maintain. Further the transducer when needed for a field experiment, does not possess the ruggedness to withstand transit movements. The device of this disclosure also has high hysteresis due to its very structure.
Andeen, et al, in Rev. of Sci. Instruments, Vol. 42, PP 495, (1971), disclose the use of ionic crystals as pressure sensing elements when formed as capacitor in sandwich structure. The pressure measurement is based on the principle of change in capacitance with applied pressure, of the capacitor structure with the material as dielectric medium between two electrodes. However the materials reported showed a larger change in capacitance by a change in temperature (temp. coefficient=250 ppm/° C.) and low pressure coefficient (−38 ppm/MPa). As a result, the materials disclosed serve more as temperature sensors rather than pressure sensors.
Kamlesh K. Jain and Subhash C. Kashyap in ‘High Temperature and High Pressures’ Vol. 27/28 pp 371 (1995), disclose the use of bismuth germanium oxide. It is disclosed that pressure coefficient and temperature coefficient of capacitance are 100 ppm/MPa and 60 ppm/° C. respectively. This is an indication of the utility of variation of capacitance with pressure as a means to measure pressure. The reliability is guaranteed to a certain extent due to low temperature coefficient but not to a level of being used as a pressure gauge.
Yet another material has been disclosed by M. V. Radhika Rao, et al in J Material Science Letter Vol. 12, pp 122 (1997). The material disclosed is a relaxor material with the following composition: 44% Lead Iron Niobate, 44% Lead Zirconium Niobate and 12% Barium Titanate. The pressure coefficient of this complex was observed to increase but without any significant decrease in temperature coefficient thereby again rendering the material not worthy of being used as a pressure transducer with capacitance parameter. Typical sintering process parameter as temperature: 900° C. The pressure coefficient was 430 ppm/MPa while temperature coefficient was +0.002/° C. Thus, the said relaxor material does not have much use as a pressure transducer.
The general draw back in all the prior art disclosure, is, therefore, low accuracy, limited usable pressure range, dependence on the need to maintain precise temperature of the transducer, and hysteresis.